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Once the must is in a fermentation vessel, yeast naturally present on the skins of the grapes, or in the environment, will sooner or later start the alcoholic fermentation, in which sugars present in the must are converted into alcohol with carbon dioxide and heat as by-products. This practice is by no means universal and is perhaps more common in New World winemaking countries. The idea is that color and fruit flavours are extracted into the aqueous solution, without extraction of tannins which takes place in post-fermentation maceration when alcohol is present. Some winemakers prefer to chill the must to around 10☌ (50☏), to allow a period of pre-fermentation maceration ("cold soaking"), of between one and four days.
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Tannin can be added to help stabilize colour, to prevent oxidation, and to help combat the effects of rot. Tannin may be added now, later in the winemaking process, or not at all. Macerating enzymes (for instance glucanases) may also be added at this stage, to aid extraction of color and fruit flavours from the skins and to facilitate pressing.
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The purpose is to prevent oxidation and sometimes to delay the onset of fermentation. The addition rate varies from zero, for perfectly healthy grapes, to up to 70 mg/litre, for grapes with a high percentage of rot. The preservative sulfur dioxide is commonly added when grapes arrive at the winery. In common with most modern winemaking equipment, destemmers and crushers are normally made of stainless steel (food-grade stainless steel for those parts which come into physical contact with the grapes). The must is then pumped to a vessel, often a tank made of stainless steel or concrete, or an oak vat, for fermentation. The mixture of grapes, skins, juice and seeds is now referred to as must. Crushers usually consist of a pair of rollers, and the gap between them can usually be regulated to allow for light, hard or no crushing, according to the winemaker's preference. Grapes pass through the holes in the cage, while stems and leaves are expelled through the open end of the cage.Īfter destemming, the grapes are commonly lightly crushed. Within this cage is a concentric axle with arms radiating towards the inner surface of the cage. Mechanical de-stemmers usually consist of a rotating cage perforated with grape-sized holes. The presence of stems during fermentation can lead to a bitter taste in the wine, and the purpose of destemming is to separate grapes from the stems and leaves. On arrival at the winery there is usually a mixture of individual berries, whole bunches (particularly with hand-picked grapes), stems, and leaves. Hand-picked or machine-harvested grapes are usually tipped into a receival bin when they arrive at the winery and conveyed by a screw mechanism to the grape-processing equipment. The first step in red wine production, after picking, involves physical processing of the grapes.